ROOT: Add Python as a dependency to assure PyROOT bindings are correctly linked.

ROOT currently does not have a :python dependency, which causes ROOT to
link against system python instead of homebrew python.  This commit adds
a :python dependency, which does not affect users who use system python
(since homebrew will still link them to system python), but which does
fix linking for users who use a brewed python.  The logic behind making
this dependency non-optional is:

1) Speaking from first-hand experience, most LHC analyses use PyROOT
more than ROOT's built-in CINT interpreter, so it makes sense to have
this always-on, as users will expect it.
2) For users who don't have a brewed python, this doesn't add any extra
build dependencies.
3) The existing formula is compiled with the --all flag, and has no
option to disable python bindings, so this really doesn't change the
interface to the formula.